In my parent application cross-referenced above, I described and claimed milling aids which could be used in a process for milling particulate aluminum in the presence of an inert liquid to produce pyrophoric type surfaces which are chemically reactive and thus can be used in the formation of aluminum compounds such as, for example, organo aluminum compounds such as aluminum alkyls or the like.
I pointed out in the background of my parent application that particulate aluminum is sometimes milled in an inert atmosphere in the presence of inert hydrocarbons such as hexane, kerosene, benzene, mineral spirits, or the like, to avoid excessive formation of an oxide-coated surface and to produce aluminum which would then combine under pressure with hydrogen gas and an olefin such as ethylene or isobutylene to form an alkylaluminum compound. In all instances that I am aware of, however, such prior art milling practices have been carried out in the presence of aluminum alkyls, which comprise a major proportion of the initial reaction medium wherein the milled aluminum particles with their fractured oxide coatings are made to react under elevated temperature and pressure according to the following scheme, depicted for triethylaluminum: EQU 2Al + 3H.sub.2 + 4(C.sub.2 H.sub.5).sub.3 Al.fwdarw. 6(C.sub.2 H.sub.5).sub.2 AlH (1) EQU 6(C.sub.2 H.sub.5).sub.2 AlH + 6C.sub.2 H.sub.4 .fwdarw. 6(C.sub.2 H.sub.5).sub.3 Al (2) EQU 2Al + 3H.sub.2 + 6C.sub.2 H.sub.4 .fwdarw. 2(C.sub.2 H.sub.5).sub.3 Al (3)
This initial stage of commercial alkylaluminum production, as indicated by reaction (1), involves dissolution of metallic aluminum in alkylaluminum product recycled from a later stage in production [reaction (2)] to form dialkylaluminum hydride. The latter compound is then combined with an olefin to generate trialkylaluminum, indicated by reaction (2). As can be seen in reactions (1) and (2), six molecules of alkylaluminum product are formed for every four molecules present in the initial reaction stage, so that the overall stoichiometry depicted as reaction (3) is a net production of alkylaluminum from aluminum, hydrogen and olefin. Thus, because aluminum alkyls are required in the reaction mixture, they are commonly introduced into the mill (in those cases where milling is the preferred method to activate the aluminum particles by fracturing the oxide protective coating) where they also serve as a carrier liquid for the aluminum particles. In no instances am I aware that anyone practices the milling of particulate aluminum under an inert liquid condition in the absence of alkyl compounds, perhaps because it is known that in such instances the milled particles will merely weld back together again thus defeating the milling operation if the purpose of the milling operation is to provide pyrophoric surfaces.
However, during the course of my experimentation with the use of milling aids, I discovered that this welding phenomena could be used and controlled to provide control of the size and shape of the particle as well as control of the particle size distribution. For example, if particles of a given size are desired, this could be produced conventionally by milling larger particles down to approximately that particle size range. However, such conventional milling practices (usually under oxidizing condition) result in the production of a large amount of undesirable fines which not only waste material but create another separation problem as well.